28 



BIOLOGIC BASIS OF SEX 



for prune. The gene responsible was found 

 to be a dominant, prune killer, K-pn, which 

 was located in the end of the 3rd chromo- 

 some at 104.5. Prune killer was without 

 phenotypic effect so far as could be ob- 

 served except that it acted as a killer for 

 flies containing any known allele of prune. 

 It was equally effective in either males or 

 females, hemizygous or homozygous, for 

 prune. The larvae died in the 2nd instar 

 but no gross abnormalities were detected 

 in the dying larvae. This case has interest 

 for the Ne gene in that the killer gene oc- 

 cupies a locus in the 3rd chromosome al- 

 though removed by about 70 units from Ne, 

 and acts on a specific genotype, the prune 

 genotype, whereas Ne acts on the specific 

 XX genotype. The known base for action 

 of the prune killer is genetically much nar- 

 rower than that for the female killer, Ne, 

 in that prune killer acts on an allele within 

 a specific locus in the X chromosome, and 

 Ne acts on a type coming as a product of 

 the action of two whole sex chromosomes. It 

 is, of course, conceivable that when ulti- 

 mately traced each action may be dependent 

 upon specific changes of particular chemi- 

 cal syntheses. The action of these genes is 

 also of interest from another viewpoint. 



In mice there is a phenotype caused by 

 the homozygous condition of a recessive 

 gene (Hollander and Gowen, 1959) which 

 acts on its own specific dominant allelic 

 type in its progeny so as to cause an in- 

 creased number of deaths between birth 

 and two weeks of age, as well as causing the 

 long bones to break and the joints to show 

 large swellings. The lethal nature of this 

 interaction is not a product of the mother's 

 milk nor does it show humoral effects such 

 as those observed with erythroblastosis in 

 the human. The interallelic interaction is 

 sex limited in that it is confined to the 

 mother and is without effect when the male 

 has the same genotype. 



H. FEMALE-MALE SEX RATIO INTERACTIONS 



A case in D. affinis involving the interac- 

 tion of a "female sex ratio" factor and an 

 autosomal "male sex ratio" factor has been 

 studied by Novitski (1947). Starting from 

 stock which had a genetic constitution for 

 the "sex ratio" X chromosome which ordi- 

 narily causes males carrying it to produce 



only daughters, he was able to establish a 

 recessive gene in chromosome B in whose 

 presence only male offspring resulted. The 

 genetic constitution of the male was alone 

 important. High sterility accompanied the 

 "male sex ratio" males breeding perform- 

 ance. The "male sex ratio" parents yielded 

 95 fertile cultures having an average of 25 

 individuals per culture. Females appeared 

 in 10 of these cultures with an average of 3 

 per culture for those producing females. The 

 total sex ratio was 77 males per female. The 

 males were morphologically normal. They 

 carried an X chromosome of their mothers. 

 Cytologic observation of spermatogonial 

 metaphases of 3 progeny showed that the 

 Fi males may or may not have carried a Y 

 of their fathers. This agreed with the tend- 

 ency for sterility in these "male sex ratio" 

 males. The ventral receptacles of the fe- 

 males from 4 such sterile cultures when ex- 

 amined proved devoid of sperm. The occa- 

 sional female offspring of the "male sex 

 ratio" males had one X chromosome from 

 each parent. Females mated to "male sex 

 ratio" males show large numbers of sperm 

 in the ventral receptacles (7 out of 8 cases) , 

 although such females had usually produced 

 no or very few offspring. The sperm, if 

 capable of fertilization, must have had a 

 lethal effect on the zygotes. The recessive 

 nature of the factor in chromosome B indi- 

 cated that its potential lethal effect origi- 

 nated during spermatogenesis rather than 

 at the time of fertilization. This lethal pe- 

 riod corresponds to that when the "female 

 sex ratio" factor in the X chromosome is 

 active. 



The research on sex ratio in Drosophila 

 reviewed shows that through the interplay 

 of the sex chromosome-located and auto- 

 somal-located factors all types of sex ratios 

 from only females in the family to only 

 males in the family may be generated. 



V. Sex Determination in Other Insects 



Sciara, a fungus gnat, offers sex-deter- 

 mining mechanisms quite different from any 

 yet offered in Drosophila. Sciara is unique, 

 yet in its uniqueness, it illustrates basic 

 facts that were rediscovered in other spe- 

 cies only through the study of abnormal 



